The invention relates generally to an apparatus and method for the passivation of stainless steel wire. In particular, the apparatus and method of the present invention electrochemically accelerate the process of chemical passivation of stainless steel wire in order to speed the process to the point that passivation can be performed on continuous long lengths of wire.
Currently, the passivation of stainless steel articles, such as a spool of stainless steel wire, is a chemical batch process which involves totally immersing the article to be treated in an acid, such as a 20% nitric acid bath for 20 minutes or more, as illustrated in FIG. 1. A spool 10 of stainless steel wire 12 is completely immersed in a nitric acid bath 14. The batch process in FIG. 1 is not particularly effective, because it takes too long and because most of the stainless steel wire 12 is shielded from the nitric acid bath 14 by the outermost wraps of the stainless steel wire around the spool 10. As a result, a majority of the stainless steel wire 12 is not exposed to the nitric acid bath 14. One other problem with the batch process illustrated in FIG. 1 is that the nitric acid itself will corrode the stainless steel wire 12 if left for long periods of time. Because of the spool arrangement, it is difficult to remove all of the nitric acid from within the wraps of the stainless steel wire.
The purpose of a passivation process is to chemically clean embedded iron and/or non-metallic inclusions from the surface of a machined metal article, in order to optimize corrosion resistance of the article. These embedded iron and non-metallic inclusions are likely sites for pitting and crevice corrosion of stainless steel in seawater. A typical stainless steel wire 12 is illustrated in FIG. 2. The stainless steel wire typically has a radius of 0.013 to 0.050 inches. During the process of manufacturing the stainless steel wire, iron particles 16 may be either embedded or smeared on the stainless steel wire 12. These iron particles can serve as initiation sites for pitting corrosion, wherein the size of the pit 18 could be as large as the radius of the stainless steel wire 12 itself. Additionally, the stainless steel wire 12 may be subject to other elements, such as chloride ions 20, which also cause crevice corrosion of the stainless steel wire 12. Both pitting and crevice corrosion may lead to mechanical failure of the stainless steel wire 12.
Stainless steel armor wires in marine electromechanical or optical cables are especially vulnerable to mechanical failure after pitting or corrosion, due to the small diameter of the wires, which is on the same order of magnitude as the corrosion pits themselves. A cross-section of an electromechanical/optical cable 30 is illustrated in FIG. 3. The optical or other data cables 32 are located at the center of the electromechanical/optical cable 30 and are covered with a water block 34. Stainless steel wire 12, acting as a strength member, surrounds the water block 34 and the stainless steel wire 12 is further surrounded by an external covering 36. One problem with the electromechanical/optical cable 30 illustrated in FIG. 3 is that crevice corrosion may form between the stainless steel wire 12 and the water block 34.
The present invention solves these problems with conventional passivation techniques by electrochemically accelerating the process of chemical passivation of stainless steel. The present invention permits rapid electrochemical removal of embedded or surface iron contaminants and removes as many possible corrosion sites to thereby enhance the corrosion resistance of stainless steel armor wires used for electromechanical cables for seawater applications. The electrochemically accelerated passivation techniques of the present invention sufficiently reduce the time required in the acid bath to allow continuous reel-to-reel process to passivate very long armor wires.
Electrochemically accelerated passivation enables passivation to be performed as a reel-to-reel process on long lengths of stainless steel wire. The process removes embedded iron and/or surface iron contamination, sites of more likely corrosion damage in sea water. Long lengths of stainless steel wires have not typically been passivated, since the conventional batch process is not conducive to multi-layer spools of wire and the geometric shielding of the inner wraps of wire. Continuous passivation of wire enhances the corrosion resistance of stainless steel armor wire used for electromechanical cables for sea water applications.